This invention relates to a battery with a collector plate welded to an electrode assembly and with improved high-rate discharge characteristics.
An exploded view of a collector plate connected to an electrode of an electrode assembly is shown in FIG. 1. The battery shown in FIG. 1, containing an electrode assembly 4 and a collector plate 6, has improved high-rate discharge characteristics and is suitable for high current discharge. In the electrode plate shown in the expanded view of FIG. 2, a lead 26A is welded to one part of the electrode. In this type of structure, all electric power collects at, and all current passes through the lead 26A. Therefore, it is difficult to improve high current discharge characteristics for the structure of FIG. 2. In the electrode assembly 4 shown in FIG. 1, an electrode upper edge connects with the collector plate 6 at a plurality of locations, and the distribution of current over the entire electrode is made uniform.
The cross-section view of FIG. 3 shows a battery with a collector plate 36 connecting to the upper end of an electrode assembly 34. In a battery with this configuration, the bottom surface of the collector plate 36 connects to one of the electrodes at a plurality of locations. To enable one of the electrode plates to connect to the collector plate 36, one of the electrodes projects out in the upward direction more than the other electrode. The projecting region of the electrode is a connecting band 37, which is a band of the electrode substrate 39 that projects out of the electrode assembly.
As shown in FIG. 4, this type of electrode assembly 34 has the collector plate 36 pressed onto its upper edge, and is connected to the collector plate 36 by resistive electric welding. For connection of the electrode plate to the collector plate 36, one region of the substrate 39 has no active material loaded or has the active material removed from a band which projects out from the electrode assembly 34, and this connecting band 37 connects to the collector plate 36. The substrate 39, which is connected to the collector plate 36, is a porous metallic material such as foamed nickel to insure that active material contacts the most substrate area.
Connection of the porous metal substrate to the collector plate in an ideal fashion is extremely difficult. Technology to accomplish this is cited in the following bulletin disclosures:
(1) Japanese Patent Publication No. 61-61230 issued on Dec. 24, 1986; PA1 (2) Japanese Non-examined Patent Publication No. 62-139251 issued on Jun. 22, 1987; PA1 (3) Japanese Non-examined Patent Publication No. 63-4562 issued on Jan. 9, 1983; and PA1 (4) Japanese Non-examined Patent Publication No. 2-220365 issued on Sep. 3, 1990.
Disclosures (1) and (2) describe structures which compress a region of substrate, such as foamed nickel, which has no active material applied to that region, to form a high density connecting band. This high density connecting band is connected to the collector plate. The battery described in disclosure (1) is provided with a high density connecting band formed by compressing the substrate together in the thickness direction. The battery of disclosure (2) is provided with a high density connecting band formed by compressing the substrate together in the height direction.
Disclosures (3) and (4) describe structures which weld attach a thin metal plate to the connecting band, which is a substrate region with no active material loaded, and this region is connected to the collector plate.
However, even with the technology described in the above disclosures, the connecting band of the substrate cannot be connected to the collector plate in an ideal fashion. In particular, to reliably connect the connecting band to the collector plate at a plurality of locations, the collector plate must be pressed onto the connecting band with considerable pressure, and attached by resistive electric welding. If the collector plate is pressed onto the connecting band with less pressure, electric resistance at connecting points between the collector plate and the connecting band becomes large, and resistive electric welding cannot be performed properly. The welding apparatus supplies a constant current to perform a weld. If the collector plate and connecting band are welded by resistive electric welding in a high resistance state, the welding apparatus must apply a high voltage between the collector plate and connecting band. If high voltage is applied, an arc discharge can occur between the collector plate and connecting band. This results in a sudden reduction in resistance, supply of high electric power to the weld region, and instantaneous melting and flying off of material at the contacting region. Proper connection of the collector plate and connecting band can in no way be accomplished with this "blast-away" condition.
To avoid this problem, the collector plate can be pressed strongly against the connecting band and welded. When this is done, the connecting band 37 bends as shown in FIGS. 5 and 6, and becomes the cause of internal short circuits. This is because the bent part breaks through the separator 33 and comes in contact with the other electrode. In particular, the discontinuous part of the connecting band 37 is weak, and the region at the border of active material loaded is easily bent. This problem cannot be eliminated either by welding a laminate thin metal plate to the connecting band 37 or by compressing the substrate to form a high density region. This is because the substrate has a discontinuous border region. In particular, a non-sintered type electrode of active material loaded into a porous metal substrate 39 is mechanically weak and is easily damaged when strongly pressed.
Further, in a battery with the collector plate welded to the connecting band at a plurality of locations, all connecting points cannot be pressed together for welding with the same pressure. For a collector plate provided with a plurality of holes with hole edges projecting downward, the projecting edges are put in contact with the connecting band for electric welding. However, considerable pressure is required against the connecting band to insure proper welding of all connecting points. Consequently, the connecting band is pressed strongly at some locations which are easily bent, and it has the disadvantage that this can easily cause internal short circuits. This disadvantage is the same for a collector plate with a flat bottom surface as well.
This invention was developed to eliminate these type of disadvantages. A primary object of the present invention is to provide a battery that connects the collector plate and the connecting band in the closest to ideal manner.